Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for electromagnetic (EM) measurements using an EM receiver that is towed above ground with a carrier.
Discussion of the Background
EM surveying is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry. EM surveys may be based on a controlled source that sends EM energy waves into the earth, which induces eddy current in the earth. The eddy currents generate a secondary EM field or ground response. By measuring the secondary field with an EM receiver, it is possible to estimate the depth and/or composition of the subsurface features. These features may be associated with subterranean hydrocarbon deposits.
A schematic airborne EM survey system 100 generally includes, as illustrated in FIG. 1, a transmitter 102 for generating a primary electromagnetic field 104 that is directed toward the earth. When primary EM field 104 enters the ground 108, it induces eddy currents 106 inside the earth. These eddy currents 106 generate a secondary electromagnetic field or ground response 110. An EM receiver 112 then measures the response 110 of the ground. Transmitter 102 and receiver 112 may be connected to an aircraft 114 so that a large area of the ground is swept. Receiver 112 may be located concentric with transmitter 102. The currents induced in the ground are a function of the earth's conductivity and, of course, the transmitter characteristics. By processing and interpreting the received signals, it is possible to study and estimate the distribution of conductivity in the subsurface. The distribution of conductivity is associated with the various layers 116 and 118 making up the subsurface, which is implicitly indicative of the location of oil and gas reservoirs and/or other resources of interest for the mining industry.
In EM geophysics, the signals of interest sometimes have low, if not ultra-low, frequency. For example, the frequency spectrum of electromagnetic signals ranges from the excitation frequency (e.g., 25 Hz) to over 100 kHz. High-frequency energy occurs a short time after the transmitter's excitation. However, at later times, low-frequency energy exists. The exploration depth associated with an EM survey system depends on the low-frequency ground response. However, during an EM survey, low-frequency motion noise is generated. If the motion-induced noise is too strong, the recorded signals may become useless from an exploration point of view. The EM survey system's accuracy depends on the ability to separate low-frequency motion noise from low-frequency ground response.
EM receivers in an EM survey system are also sensitive to motion-induced noise as the receiver moves through the earth's magnetic field (noise due to towing the coil above ground). Motion-induced noise may be several orders of magnitude larger than the ground response at low frequencies.
While some efforts have been made to directly measure and compensate for receiver coil motion (see, e.g., U.S. Pat. No. 6,876,202, the entire content of which is incorporated herein by reference) to improve the exploration depth for both passive and active airborne EM surveys, most contractors have developed sophisticated suspension systems to isolate the receiver coil from various motion-induced noise (i.e., towing noise) sources, which include: buffeting of the enclosure as it moves through turbulent air, natural resonance of the receiver's supporting structure, motion of the aircraft as it adjusts to maintain its target flight path, and mechanical vibrations of the aircraft and engines.
A typical receiver coil suspension system is implemented with either one or two stages of elastic members connecting the receiver coil to the outer structure. In this regard, see, for example, FIGS. 2A and 2B corresponding to FIGS. 4A and 2A, respectively, of U.S. Pat. No. 8,362,779, the contents of which are incorporated herein by reference. FIG. 2A is an overall view of a receiver coil frame 24 that includes a frame 12 for supporting receiver coil 14 and rings 16, while FIG. 2B is a cross-section showing how receiver coil 14 is suspended with inner suspension members 10 from frame 12, and frame 12 is suspended with suspension members 20 from rings 16. This design includes two stages of elastic members supporting a single axis receiver coil 14.
However, the two-stage suspension illustrated in FIGS. 2A and 2B doubles the high-frequency attenuation roll-off, at the expense of introducing additional low-frequency modes that cause the low-frequency behavior to deteriorate. In this regard, FIG. 3 compares the response 300 of a receiver coil having a single-stage suspension system to the response 302 of a receiver coil having a dual-stage suspension system, both normalized to the electronic noise floor 304 for typical airborne EM receivers. The electronic noise floor 304 represents the noise induced by the electronics associated with the receiver coil. Any response that is above electronic noise floor 304 is unusable for determining the configuration of the earth beneath its surface. The single-isolation stage is superior at frequencies below 5 Hz. However, the dual-isolation stage falls below the electronic noise floor at approximately 25 Hz, meaning that measurements above 25 Hz can be used to characterize the subsurface.
A relationship between EM receiver isolation and electronic noise floor defines the lowest practical base frequency for a given EM system, limiting all known systems to approximately 25 Hz (i.e., traditional EM systems cannot produce a useful signal below 25 Hz). Since depth of exploration and base frequency are directly related, a more advanced suspension system is desirable to allow base frequencies throughout the Extremely Low-Frequency (ELF) band of 3 Hz to 30 Hz to significantly improve overall depth of exploration of EM systems. Thus, there is a need to develop a new suspension system that allows base frequencies throughout the ELF band.